Electric Forces & Potential: every key term you need (+ practice quiz)
24 flashcard terms for AP Physics 2 Unit 3, written to match the course framework. Study them here, then drill them as interactive flashcards, or test yourself with the 24-question quiz — free, no account needed.
F = k·q₁q₂/r² where k = 8.99×10⁹ N·m²/C². Electric force between charges; repulsive if same sign, attractive if opposite.
Electric Field
E = F/q = kQ/r². Force per unit positive charge. Vector; points away from positive, toward negative charge.
Electric Potential
V = U/q = kQ/r. Energy per unit positive charge. Scalar. Equipotentials: points of same potential.
Electric Potential Difference
ΔV = V_B - V_A. Work per charge moving from A to B. W = qΔV.
Relationship: E & V
E = -dV/dx. Electric field is negative gradient of potential. Higher field = steeper potential change.
Uniform Electric Field
E constant between parallel plates. V = Ed where d = plate separation. Field points from + to -.
Potential Energy
U = kq₁q₂/r. Repulsive (q same sign) = positive U. Attractive (q opposite) = negative U.
Conductor vs Insulator
Conductor: charges move freely (metal). Electric field = 0 inside. Insulator: charges fixed (rubber). Field passes through.
Electron Volts (eV)
Energy unit: 1 eV = work to move 1 electron across 1 volt potential difference. 1 eV = 1.6×10⁻¹⁹ J.
Unit 3 Summary
Coulomb's law governs electric force. Electric field: force per charge. Potential: energy per charge. V and E related.
Superposition of Fields
Net E at a point is the vector sum of fields from each charge. Between two equal positive charges the field is zero at the midpoint; between a +q and −q it points from + to − and never vanishes between them.
Zero-Field Point for Unequal Charges
For a +4q and +q separated by d, E = 0 lies on the line between them, closer to the smaller charge: k(4q)/x² = kq/(d − x)² gives x = 2d/3 from the 4q charge.
Potential Is a Scalar
V = Σ kq_i/r_i adds algebraically with signs; no components. Between +q and −q, V = 0 at the midpoint even though E is not zero there — potential and field are independent checks.
Field from Potential Gradient
E points from high V to low V with magnitude |ΔV/Δd| in the steepest direction. Closely spaced equipotential lines mean a strong field; E is always perpendicular to equipotentials.
Work by Field vs by External Agent
W_field = −qΔV = −ΔU. Moving a positive charge to higher potential requires positive external work; the field does negative work. Sign errors here are the most common FRQ deduction.
Potential Energy of a Charge System
U_total = Σ over pairs of kq_iq_j/r_ij. Three charges have three pair terms. Assembling like charges from infinity stores positive energy; unlike charges release energy.
Conductor in Electrostatic Equilibrium
E = 0 inside the metal, all excess charge on the surface, surface is an equipotential, E just outside is perpendicular to the surface and largest where curvature is sharpest (points).
Charge by Induction
Bring a charged rod near a neutral conductor, ground it, remove ground, then remove rod: conductor is left with charge opposite to the rod's. No contact needed; the rod loses no charge.
Molecules in an insulator stretch or align so a charged object attracts a neutral one (paper to a comb). Net charge stays zero, but the near side is closer, so attraction wins over repulsion.
Motion in a Uniform Field
A charge in uniform E accelerates at a = qE/m; a perpendicular entry gives a parabolic path exactly like projectile motion, with qE replacing mg. Electrons curve much more sharply than protons.
Energy Conservation with Charges
½mv² + qV = constant when only electric forces act. A proton released from rest across ΔV = 1000 V gains 1000 eV = 1.6 × 10^-16 J regardless of the path or field shape.
Field Lines Rules
Start on +, end on −, never cross, density ∝ |E|. Number of lines from a charge scales with its magnitude; a 3q charge has three times the lines of q.
Shell Theorem (Physics 2 level)
Outside a uniformly charged sphere or shell, E and V behave as if all charge sat at the center. Inside a conducting or hollow charged shell, E = 0 and V is constant (equal to its surface value).
Point Charge Field vs Potential Falloff
E ∝ 1/r² but V ∝ 1/r. Doubling the distance from a point charge quarters E but only halves V — a favorite ratio question.